The present invention relates to a sealed type motor-operated flow control valve.
Motor-operated flow control valves of this type are disclosed in Japanese Utility Model Disclosure Nos. 24576/84 and 153774/83.
In a sealed type motor-operated flow control valve disclosed in Japanese Utility Model Disclosure No. 24576/84, one end portion of a rotating shaft of a rotor is screwed in a valve body which includes a first passage, a second passage, and a valve port connecting the first and second passages and formed with a valve seat. A valve member is integrally coupled to the end face of the rotating shaft at the one end portion thereof. The rotor is housed in a casing which is hermetically coupled to the valve body. A radial/thrust bearing provided on the inner surface of the casing supports the other end portion of the rotating shaft of the rotor. A stator is provided on the outer surface of the casing. In the prior art sealed type motor-operated flow control valve of this construction, the valve member is moved relatively to the valve seat between an open position and a closed position by rotating the rotor in one or the other direction. Since the wall of the casing is interposed between the rotor and the stator, however, an electric motor consisting of the rotor and the stator is low in operating efficiency, suffering a relatively great magnetic loss. With the low motor efficiency, the stator is required to be large-sized in order to provide the necessary torque for the drive of the valve member. This inevitably leads to an increase in overall size of the control valve. In this prior art control valve, moreover, the radial/thrust bearing is fixedly supported on the inner surface of the casing in a preassembly, so that heat produced during welding work for hermetically coupling the casing to the valve body may sometimes distort the casing to deflect the center of the radial/thrust bearing. If the center is deflected, the rotor will suffer greater rolling friction or be disabled from rotating. In the end, it is impossible to control with a desired accuracy the rate of flow of a fluid passing through the control valve. This constitutes a hindrance to reduction of the fraction of defective products in the production line for control valves.
In a sealed type motor-operated flow control valve disclosed in Japanese Utility Model Disclosure No. 153774/83, a valve member is screwed in a valve body which incldues a first passage, a second passage, and a valve port connecting the first and second passages and formed with a valve seat. A rotating shaft of an electric motor is coupled to that end portion (proximal end portion) of the valve member more distant from the valve seat for axial slide and for transmission of circumferential rotatory force. The motor is formed as an independent unit in which a stator and a rotor are contained in a housing, and which is independently available. The motor is housed in a casing hermetically coupled to the valve body and is supported by the inner surface of the casing. In the prior art sealed type motor-operated flow control valve of this construction, the housing of the motor as an independent unit is covered with the casing. Therefore, the external dimensions of the control valve should inevitably be increased.
In the control valve stated in the former Japanese Utility Model Disclosure, the one end portion of the rotating shaft of the rotor is screwed in the valve body. In the control valve disclosed in the latter, on the other hand, the valve member is screwed in the valve body. These arrangements will inevitably cause a backlash between the mating regions of male and female screws, so that the valve opening strictly is not constant even while the rotor is not rotating. Thus, it is impossible to control the flow rate of the fluid very accurately.